1. Field of the Invention
This invention relates to a pneumatic radial tire having a reduced rolling resistance and particularly suitable for a passenger car.
2. Description of the Prior Art
In general, a radial tire comprises a carcass body reinforcement which consists of a toroidal carcass composed of plies including radially arranged cords, that is, cords arranged in a radial plane inclusive of a rotary axis of the tire or a plane inclined at a small angle to the radial plane and crossed therewith. Also a plurality of belt layers are superimposed about the crown portion of the carcass and include cords inclined at an angle of a range between 60.degree. and 80.degree. to the radial plane of the tire, the cords of adjacent belt layers being crossed with each other. A bead reinforcement consists of the lower portion of the toroidal carcass wound around a bead core to form a turn-up portion. The carcass body reinforcement and bead reinforcement are covered with outer rubber layer of the tread portion and side portion made integral with the bead portion by vulcanization.
The rolling resistance of the tire produced when a vehicle provided with the above mentioned radial tire runs on a flat paved road without inclination is produced mainly due to the internal loss caused by flexure of the tire formed during rotation of tire and the frictional loss between the tire and the road surface. In this case, the fact that the rate of the internal loss taken in the above two losses is large has been well known in the art. Many attempts have been made to use a material having a relatively low loss modulus of elasticity as the rubber for all portions exclusive of the cords.
But, the tire must display different abilities in dependence with its different positions such as the tread portion, side portion or the like. As a result, if, for example, the rubber having a low loss modulus of elasticity is applied to the tread portion and is maximized in amount, hence tending to give a maximum contribution to the ability of the tire, the frictional coefficient with the road surface considerably decreases to deteriorate the anti skid property and breaking property, thereby restricting the safety of the tire.
Means for reducing the rolling resistance of the tire comprising an annular groove arranged in the outer rubber layer of the shoulder of the tire and circumferentially extending along the tire has been proposed and described in U.S. Pat. No. 3,253,635. In this tire, the annular groove is operative to interrupt the tread portion and the side portion which are different in operation as an oscillatory system of the tire when it runs from each other. Such kind of tire, however, is the same in construction as a tire including nibbling groove which has been well known in the art and hardly displays an effect of reducing the rolling resistance. The tire including a nibbling groove is a tire which is provided at its shoulder portion with a deep annular groove circumferentially extending the tire and operative to easily deform transversely the tread portion side edge for the purpose of reducing the resistance that tends to be produced when the tire rides over in the lengthwise direction of projections slightly raised from the road surface such as a rail for street car provided along the road.
The rolling resistance of the tire exerts a direct influence upon the driving force of automobile, so that there is an urgent requirement in the tire manufacturing field to reduce considerable the rolling resistance of the tire in compliance with the requirements of economizing energy.